With the rapid development of nanoscience, it is necessary to develop simple and rapid fabrication techniques to create highly ordered functional nanostructures (1-3). Self-assembled monolayers (SAMs) have sparked interest, since the terminal functional group of the SAM bonded to the substrate can modify the surface properties of the bare substrate, for example, to create patterns of hydrophobic or hydrophilic characteristics (4,5). Such chemical surface structures prepared with functionalized SAMs may play an important role in the site-specific immobilization of molecules (6).
Patterned monolayers have been generated by a variety of ways, including by direct patterning or indirect patterning (7,8). The direct patterning approaches include microcontact printing and dip-pen nanolithography (DPN), in which the adsorbates are selectively placed on the surface and then the remaining surface area is backfilled by a second adsorbate or left bare. Previous work on patterning SAMs has focussed primarily on microcontact printing technology (9). This technology is limited, however, by the challenges of achieving high resolution (e.g., line widths of <50 nm) and replication accuracy, due to degradation of the elastomeric stamp and the unbalanced contact pressure applied to the substrate (8). DPN, as another “direct writing” technology to printing SAMs, was demonstrated by Mirkin's group (10,11). With DPN, the preparation of patterned features has been explored down to 15 nm line width and is controlled by a complex mixture of factors, such as the tip-substrate contact time, relative humidity, and scan speed (12). Unfortunately, the speed of writing with an atomic force microscope (AFM) tip as practiced with DPN is quite slow, and scale-up requires the development of multiple scanning probe tips to enable paralleling processing (13).
In indirect patterning, the molecules from a pre-existing SAM are removed, reacted or destroyed using AFM, scanning tunneling microscopy (STM) or an energetic beam, followed by back filling the bare areas by a second adsorbate (3,8). A typical example is nanografting (14). Liu et al. patterned alkanethiolate SAMs by selective removal of adsorbates either by applying a mechanical force with an AFM tip or by sending high energy electrons from the tip to the substrate using STM (15,16). Again, as with DPN, this method is limited by the slow nature of these processes. In addition, a key issue is control of the normal force, which is often difficult to adjust experimentally and is theoretically calculated. Energetic beams have also been used to destroy or react selected portions of pre-existed SAMs (17-19). For example, Sun, et al. used Ultra-violet (UV) radiation to selectively remove portions of a SAM made of 11-mercaptoundecanoic acid and then backfilling using n-dodecanethiolate to obtain a patterned SAM. Grunze and coworkers (20) irradiated a 4′-nitro-1,1′-biphenyl-4-thiol (NBT) preabsorbed gold substrate with 50 eV electrons through transmission electron microscopy (TEM) grids to reduce nitro groups in the exposed area to amino groups, and then used n-dodecanethiol to replace the NBT in the unexposed area to form patterned SAMs with different functional groups. Craighead, et al. have demonstrated patterning with bi-component SAMs with NH2— and CH3— termini using a low energy electron beam to irradiate a methyl-terminated SAM followed by backfilling the damaged areas with cysteamine generated SAMs (21). These patterning methods involving the oxidation or reduction of SAMs by an energetic beam depend, however, on the sensitivity of a particular SAM to irradiation, which limits the range of accessible chemical functionalities.
Polymeric materials, particularly block copolymers and blends, haven been investigated because of their ability to self assemble into a variety of interesting and useful morphologies (22). These morphologies can be used as flexible templates for assembly of nanodevices (23) that are appropriately modified to “mate” with the block copolymer (24,25). In order to obtain long range order in block copolymers, techniques such as nanopatterned surfaces (26,27) and electric fields (28,29) have been used. Chemically modified surfaces have also been used to prepare defect-free nanopatterns over large areas (30). Although certain approaches have been demonstrated, the methods are often unsuitable for scale-up. For example, long annealing times are typically necessary, making the approach impractical for high rate, high volume processing methods. In order to make this approach viable for commercial products, the ability to obtain the desired structure over large areas in a high rate, high volume process is required. Additionally, many approaches focus on patterning of block copolymers; however, this approach is limited in that the pattern spacing is dictated by the polymer structure.
Another previously described technique involves the patterning of silane monolayers (34). However, that method is limited by the high electron sensitivity and chemical and physical stability required of the PMMA resists in the silanation process, which must be stable to high temperatures and long reaction times.